In cellular communications networks, it is important to improve the throughput that can be achieved by the network. However, it is also important to improve the satisfaction of users of the network by improving the performance that they experience. It is particularly important to improve the performance experienced by user equipments (UEs) at the edge of cells in the network. The reason for this is that UEs close to the centre of cells in the network are generally happy with the performance of the network, and further improvement to the performance experienced by these UEs would proportionally improve their satisfaction. However, it would require more effort to improve the performance experienced by UEs at the edge of cells in the network since these UEs would typically be less happy with the performance of the network. Therefore, a significantly greater overall satisfaction of users in the network could be achieved by improving the satisfaction level of users at the edge of cells in the network.
In addition to improving the satisfaction levels of users of the cellular communications network, it is also important to employ measures for saving energy in the network. If energy is saved in a network, the amount of expenditure and operating costs for the network is reduced and, moreover, the impact on the environment (which has become an increasing concern due to rapidly rising energy consumption) is also reduced.
Further to this, it is also important to ensure that basestations in the cellular communications network are able to communicate with each other effectively. The 3rd Generation Partnership Project (3GPP) provides a means for evolved Node Bs (eNBs) to communicate with each other. However, it is not often practical or the option is not always available in many deployment scenarios for a network to achieve direct communication between two eNBs. For example, this is true of a heterogeneous network (HetNet) in which cells of different sizes, different vendors, or even different technologies are deployed in potentially overlapping regions.
In this type of network, it is possible for a large macro cell to convey information to smaller pico cells for interference purposes. However, it is rarely possible for a small pico cell to convey information to larger macro cells for interference purposes. This results in an asymmetric communication between cells, which makes the effective coordination of interference among cells of different sizes extremely difficult. Moreover, the small cells of the network are randomly deployed, which means that the small cells could reside both in the centre of the macro cell and at the edge of the macro cell. Therefore, signalling for Inter-Cell Interference Coordination (ICIC) purposes over a standard interface through which the basestations are connected (for example, over the X2-interface) requires more refinement due to the non-uniform geometry associated with a HetNet.
Fourth Generation (4G) cellular systems such as Long-Term Evolution (LTE) systems are currently being developed in order to improve both system performance and data rates achieved for UEs compared to that achieved by Third Generation (3G) cellular systems. Although the 4G systems are designed to improve system performance and data rates achieved for UEs, it is also important to improve the performance experienced by UEs at the edge of cells in the network. One of the most effective ways to make such an improvement is via power and interference management.
Generally, power and interference management is designed to improve the overall performance of systems and the performance experienced by UEs by reducing unnecessary interference. The aim of this is to reduce as much power as possible in order to meet a certain satisfaction objective. By eliminating unnecessary transmit power, it is possible to significantly improve the energy efficiency of a network. It is noted that improvement in the energy efficiency of a single base station would not necessarily have a significant impact on the amount of money saved. However, the money saving that is achieved by improving the energy efficiency of basestations in a network having many basestations can be particularly large.
Typically, a cellular communications network comprises more than a single cell, which means that each cell is likely to be surrounded by neighbouring cells. As a UE in the network moves away from the basestation that is currently serving it (the serving basestation) towards a basestation of a neighbouring cell, the call quality experienced by that UE degrades due to a weakening in the quality of the signal received from the serving cell and due to an increase in the interference experienced from the neighbouring cells to which the UE is becoming closer. This type of interference is often referred to as inter-cell interference, and the mitigation of this interference is particularly important to improve the performance experienced by UEs at the edge of cells in the network. It is more complicated to manage interference in LTE systems than it is to manage interference in 3G systems (such as Wideband Code Division Multiple Access (WCDMA) systems) because LTE systems involve power allocation in both the time and the frequency domain whereas 3G systems only involve power allocation in the time domain.
An existing method for the mitigation of inter-cell interference in a cellular communications network is based on Fractional Frequency Reuse (FFR) in which UEs in the centre of each cell of the network are allocated the same frequency, whereas UEs at the edge of cells are allocated with a subset of frequencies that are different from those of the immediate neighbour cell. As a result, the inter-cell interference experienced by UEs at the edge of cells is reduced significantly.
However, this existing method for the mitigation of inter-cell interference suffers from many drawbacks. In particular, it is necessary to carefully plan the subset of frequencies used for the UEs at the edge of cells in the network that can be a complex and time consuming process. Typically, the subset of frequencies is allocated statically during the network planning stage. This method is especially not suitable for femto-cells in which basestations are deployed in an ad hoc manner. Furthermore, this method does not take into account the dynamic distributions of traffic in the network, which can result in the spectrum of frequencies being underutilized.
An alternative method for the allocation of power and frequency resources allocates frequency, power, modulation, and coding schemes (MCS) jointly for each UE in a cell in a centralized way. However, this type of approach requires a centralized entity, and the computation complexity is impractically high.
The published paper entitled “Self-organizing Dynamic Fractional Frequency Reuse for Best-Effort Traffic Through Distributed Inter-cell Coordination”, by A. L. Stolyar, and H. Viswanathan, in proceedings IEEE Infocomm, April 2009, discloses an existing algorithm for self-optimization of a network that aims to improve the overall capacity and/or cell edge data rates. The algorithm is gradient-based in that frequency reuse patterns are dynamically adapted based on the traffic distribution. As this approach is self-organizing among cells in a distributive manner, the time-consuming process of frequency planning is not required. Furthermore, this method not only provides a way to assign frequency in a distributive manner, it also allows the power to be adjusted dynamically in frequency, and thereby provides an extra degree of flexibility. The method allows eNBs to continuously and autonomously adjust their power based solely on standard UE feedbacks.
However, while this existing approach is useful, it does not take into account the Quality of Service (QoS) experienced by UEs. As a result, the power allocation may not necessarily be tailored to the requirements of UEs and, as a result, the power efficiency in the network may be reduced. Moreover, the information exchange required in this existing algorithm is not readily supported by the standard X2-interface. In addition, this approach does not take into account the concept of energy saving, and the power allocation may not be optimal.
Other existing algorithms have been known to take into account QoS requirements directly in power adjustment, provide a simpler message exchange among eNBs, and have been made compatible with the X2 interface. Such an algorithm is disclosed in the published paper entitled “Distributed Energy-Saving Mechanism for Self-Organizing Femto LTE Networks”, by R. Kwan, in proceedings IEEE Vehicular Technology Conference (VTC) Fall, Quebec, Canada 2012. This existing algorithm suggests that it is possible to achieve significant power saving if the minimal amount of power suitable to meet the QoS requirements is allocated to basestations of the network. Of course, energy saving is only possible if the QoS requirement is not too high for the capacity of the system. The important aspect of this algorithm is that it is possible to maintain suitable performance levels for UEs at the edge of cells in the network while simultaneously providing a suitable level of power saving.
The algorithms discussed above each require some level of communication among eNBs. However, there exist situations where the X2-interface will not always be available. One example is in a heterogeneous network (HetNet), where femto-cells, small cells, and macro cells co-exist. In a HetNet, direct communication between cells may not easily be achievable (for example, since HetNets include cells that would operate under different protocols). Another example is in the case of inter-vendor femto cell deployment. In this situation, it is not clear whether a standard X2-interface exists between femto cells belonging to different vendors. Even if a standard X2-interface did exist, there is no guarantee that each femto cell would react appropriately according to the expectation of another.
There is thus a need for an autonomous algorithm that does not require the use of a standard X2-interface. There are algorithms that already exist that make this possible. However, these existing algorithms require certain sub-band specific feedback from UEs in the network, which is not compatible with 3GPP.
The present disclosure refines the existing algorithms discussed above to take into account the notion of energy saving. In particular, the present disclosure takes into account a factor relating to the happiness of the users of the network to continually adjust the power with the aim of saving energy. The disclosure particularly concentrates on the benefits that can be achieved at the edge of cells in the network while providing continuous refinement of power with the aim of reducing power consumption in the network.
Furthermore, the present disclosure improves the performance experienced by UEs at the edge of cells in the network (thereby increasing the range of the cells) while eliminating unnecessary power consumption (through energy saving) in an autonomous fashion without any need for explicit communications with neighbouring cells. This eliminates the need to establish communication links among neighbour cells for interference the purpose of interference mitigation. In this way, direct interfaces (such as the X2-interface) among cells in the network are not required because each cell is able to autonomously perform a power allocation strategy of its own. In other words, each individual cell is able to adapt its own power to a desired level without the need for external communication. This feature of the disclosure is particularly beneficial in a heterogeneous network (HetNet) since the downlink power of each cell in the heterogeneous network would adapt itself to the desired level. In this way, the need for complex planning of network deployments is eliminated.
In summary, the present disclosure provides a method for adjusting the downlink transmit power of networks (for example, LTE networks) in an intelligent way, which does not require dedicated interfaces among basestations of the network for interference mitigation purposes, and which significantly reduces power consumption. This reduces costs while providing improved coverage at the edge of cells in the network, thereby enhancing the satisfaction of users at the edge of cells and increasing the cell range. The present disclosure achieves energy saving at the same time as improving performance at the edge of cells in an autonomous way without the need for any exchange of information among basestations.
The present disclosure is compatible with schemes where communication links exist between peer nodes as well as between nodes of different layers. It is also compatible with schemes operated at a different time scale, and in agreement with the theory surrounding the coexistence of multi-layer SON operations.
According to a first aspect of the present disclosure, there is provided a method for controlling a power allocation in a base station of a cell in a cellular communications network across a plurality of sub-bands, the method comprising the steps of:
determining whether a long-term performance achieved by the cell exceeds a first threshold value; and
allocating a total transmit power across the sub-bands according to whether the long-term performance achieved by the cell exceeds the first threshold value.
According to a second aspect of the disclosure, there is provided a basestation configured to operate in accordance with the method of the first aspect of the disclosure.